[3] In 1864 Purvis served in the Union Army in the US Civil War as a military nurse at Camp Barker, which became a model for Freedmen's hospital.

Two months after graduation he took the position of acting assistant surgeon with a rank of first lieutenant and was assigned to duty in Washington, DC.

[6] In the fall of 1868, Purvis was elected professor of materia medica and medical jurisprudence, a position which he held for five years.

[2] Purvis was very successful at Howard, and was credited with keeping the medical department running during the financial panic of 1873.

[3] Later in 1881 Purvis was appointed by President Chester Arthur as Surgeon-in-Charge at the Freedmen's hospital,[2] serving from October 1, 1881, to 1894.

[3] In a letter from 1908, Purvis said he believed he was removed by the Secretary of the Interior Hoke Smith in favor of a Democrat after a change in administrations, which was customary for political appointees under the spoils system.

[8] In Washington, Purvis was close personal friends with many prominent leaders, including Frederick Douglass, Francis J. Grimke, Blanche K. Bruce, and Richard Theodore Greener.

[9] In 1881, Purvis joined James Monroe Gregory and George T. Downing in fighting against a proposed law before the U.S. House of Representatives that would create separate schools for black children.

The group gathered many leading civil rights figures, having Frederick Douglass as president, Richard T. Greener as secretary, and also including Frederick G. Barbadoes, John F. Cook, Francis James Grimké, Milton M. Holland, Wiley Lane, William H. Smith, Purvis, Downing, and Gregory.

The group was supported by representative Dudley C. Haskell (R-KS) and succeeded in forestalling the proposal.